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Molecular structure and the occurrence of smectic A and smectic c phases
W.H. de Jeu
To cite this version:
W.H. de Jeu. Molecular structure and the occurrence of smectic A and smectic c phases. Journal de
Physique, 1977, 38 (10), pp.1265-1273. �10.1051/jphys:0197700380100126500�. �jpa-00208696�
MOLECULAR STRUCTURE AND THE OCCURRENCE OF SMECTIC A AND SMECTIC C PHASES (*)
W. H. de JEU
Philips
ResearchLaboratories, Eindhoven,
The Netherlands(Reçu
le 4avril 1977, accepté
le6 juin 1977)
Résumé. - On étudie les
propriétés mésomorphes
de divers azobenzènes substitués en para par des chaînes n-alkyles oun-alkoxy.
Lesdialkylazobenzènes
nonpolaires
présentent desphases nématiques
et smectiques A. Quand onremplace
une chaînealkyl
par une chaîne alkoxy (avec créationd’un moment dipolaire terminal), il y a augmentation de la tendance à
l’apparition
d’une phasesmectique
C. Ces résultats peuvent être interprétés par un modèle dipolaire de la phase smectique C et ne confirment pasl’hypothèse
que cette phase résulte principalement d’interactionsspatiales
entredes molécules en
configuration zig-zag.
Dans le cas d’un seul moment dipolaire terminal, l’une desdeux
possibilités
de modèle à interaction dipolaire prévoit des couchessmectiques ferroélectriques.
Cette situation
pourrait
éventuellement fournir un modèle de la phase smectique F.Abstract. - The mesomorphic properties of various terminally alkyl and/or alkoxy substituted azobenzenes are
investigated.
The non-polardialkylazobenzenes
have nematic and smectic A phases.For each alkyl group that is replaced by an alkoxy group (thus introducing an outboard
dipole
moment) the tendency to form a smectic Cphase
is increased. These results can be rationalized in terms of adipole
model of the smectic C phase, and do not support the idea that this phase occurs mainly because of steric interactions between zig-zag shaped molecules. In the case of only one out- boarddipole
moment there are twopossibilities
for a model withdipole
interaction, one of which hasferroelectric smectic layers. This situation could possibly provide a model for the smectic F phase.
Classification Physics Abstracts
61.30
1. Introduction - The various
liquid crystalline phases
are characterizedby long-range
orientationalordering [1].
Theelongated
molecules are, on average,aligned
with theirlong
axesparallel
to apreferred
direction in space. In a nematic
liquid crystal
themolecules translate
freely,
and the centres of mass aredistributed at random. Therefore the
X-ray
diffractionpattern
contains nosharp
reflections. Smecticliquid crystals,
on the otherhand,
have alayered
structure : the molecular centres are situated in a series ofequi-
distant
planes.
In theX-ray
diffractionpattern
asharp
reflection is observed
corresponding
to theinterplanar distance,
which is of the order of the molecularlength.
In the smectic A and C
phases
the distribution of the centres of mass within thelayers
is random. The w nematic(N)
and the smectic Aphase (SJ
have theoptical properties
of a uniaxialcrystal;
the smectic Cphase (Sc)
is found to be biaxial.During
the last few years much attention has beengiven
to the nature of the intermolecular forces that (*) Part of this paper was presented at the « Conference Euro-p6enne sur les Smectiques Thermotropes et leurs Applications »,
Les Arcs (France), 15-18 December 1975.
lead to the formation of an
SA phase [2-4]
or anSc phase [5-8].
A crucialquestion
is whether the interac- tion betweenpermanent dipole
moments isimportant
for the formation of the
Sc phase.
It is the purpose of this paper toprovide
a molecular basis for this dis- cussionby investigating
the type of smecticphases occurring
in some series ofcompounds
which havebeen selected because of
specific
structural differences.Section 2
begins
with a review of the various theories for theSc phase,
withemphasis
on thepresumptions
about the molecular
properties
of the constituentcompounds.
Section 3 discusses the smecticphases occurring
in variousterminally
substituted azo- andazoxybenzenes.
Thep,p’-di-n-alkylazobenzenes [9]
arek a suitable
starting point
for such acomparison
becausethey
arenon-polar. By substituting alkoxy
foralkyl and/or azoxybenzene
forazobenzene, dipole
momentscan be introduced at
specific positions
whileonly
minor variations of the molecularshape
occur. The resultsare discussed in section 4. It turns out that in these
cases the occurrence of an
Sc phase
can be understood with the aid of asimple
extension of McMillan’sdipole
model. Steric
repulsions
areprobably
not a dominanteffect. The extension of the
dipole theory
of theArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197700380100126500
1266
Sc phase
alsoprovides
apossible
model for thesmectic F
phase.
In the literature the smectic Fphase
isreported
to occur below anSc phase
uponcooling
ofsome
compounds [10].
It seems to have all the pro-perties
of theSc phase (liquid layers, biaxiality, etc.)
and the ways in which it differs from the
Sc phase
havenot yet been determined.
2. Models for the smectic C
phase.
- In all modelsfor the smectic C
phase
it is assumed that the smectic A order is well established : there is orientational order-ing
of thelong
molecular axes andpositional ordering
of the molecules in
layers.
If B denotes theangle
between the
long
molecular axis and thepreferred direction,
the molecular orientation can be describedby
a distribution functionf ’(9),
whereis the
probability
that thelong
molecular axis will forman
angle
between 0 and 6 + d0 with thepreferred
direction. The average
degree
of orientationalordering
can be described
by
an order parameter[1]
The distribution function
J’(0)
is related toV(O),
theorientation
dependent part
of thepotential, by j’(9) = (I/Z) exp(- V/kT)
where Z is a normaliza-tion constant. In Maier and
Saupe’s theory [11]
of thenematic
phase V(9)
is calculated in a mean-fieldapproximation assuming
that it comes from the ani-sotropic part
of thedispersion
forces. This leads to aset of self-consistent
equations for 17
andv(9)
that canbe solved to
give 17
versus temperature,yielding
a NI
phase
transition atTNI. V(O)
can be shown to beapproximately proportional
to thesquared anisotropy
of the molecular
polarizability [12].
In
practice
theelongated
molecules often possess a skeleton of a electrons andadditionally
a central coreof dclocalizcd vr electrons.
Consequently
thepolari- zability
is to alarge
extent concentrated in the central part of themolecule,
and therefore the molecules willprefer
to have their centralparts
closetogether.
Thiseffect becomes more
pronounced
if the skeletonof Q electrons is extended. Thus in a
homologous
seriesthe
tendency
to form smecticphases
increases withincreasing length
of the molecules. This isgenerally accepted
to be theorigin
of the occurrence of smecticphases, although
some curiousexceptions
have beennoted in the case of
strongly asymmetrically
substitut-ed molecules
[13].
McMillan has made these ideasmore
quantitative
in a model thatignores
thepolariza- bility
of the end groups, and takes a Gaussian distri- bution for the interaction between the centralparts
ofthe molecules
[3].
The modelpredicts
aNSA phase
transition that may be
second-order, depending
on theratio between the
length
of the central aromatic coreand the total
length
of the molecule. Thesepredictions
are at least
qualitatively
in agreement withexperi-
mental results.
For the
Sc phase
the literature contains various models on which there is as yet nogeneral
agreement.McMillan
[5]
assumes that aprimary
role isplayed by
interactions between transverse permanent
dipole
moments. Unlike the situation in the N and the
SA phase,
the rotation around thelong
molecular axis cannot then becompletely
free for the functionalgroups with which the
dipoles
are connected. Let us assume that theSA
order is wellestablished,
and thatthe molecules can be
represented by cylinders
with twooutboard
dipole
moments J1 at a distanced/2
from thecentre
[5] (see Fig. la).
Now we consider the interac- tion between thedipole
moments of the molecules in asmectic
plane. The’preferred
direction is takenalong
the
z-axis,
while theangle
between the x-axis and oneof the
dipoles
is denotedby
9. Then thesingle particle potential
can be written aswhere E is the field at the
position
of adipole
due to thedipoles
of the other molecules. This fielddepends
on
g(r),
thetwo-particle
correlationfunction,
which issimply
assumed to beFIG. l. - Schematic representation of the dipole model (a) and of the steric model (b) of the Sc phase.
where n2 is the
particle density
in the smecticplane.
Neglecting
the interactions betweendipoles
at differentlevels,
we can then calculate E and find that it is afunction
of fi = It#,
the average value of the upper orlower
dipole
moment. In turnIlP
is calculated self-consistently using
thepotential
ofequation (2).
As aresult a second-order
phase
transitionSA SC
is pre- dicted at[5]
where k is Boltzmann’s constant and Boo the
high- frequency
dielectricpermittivity.
In this model theSc phase
ispredicted
to be biaxialprovided
theconstituent molecules are biaxial
[5].
If the outboarddipoles
are notexactly perpendicular
to thelong
molecular
axis,
but have acomponent 6 along
thisaxis,
there is atorque
3A;7cA 6fl to
tilt the moleculeover in the x direction. There will be a
restoring
torque -K 03C8, where 4/
is the tiltangle
and K is anelastic constant.
Equating
the two torques andusing
the
explicit
resultfor P [5]
a tiltangle
is foundgiven by
Although
thephysical properties
of McMillan’s model agree well with those of theSc phase,
there is consi-derable
disagreement
on thequestion
of whether there is inreality
no free rotation of the molecules around theirlong
axis[14, 15].
It should beemphasized, however,
that the model stillpermits
rotation ofparts
of the molecule not connected with the
dipole
moments. Hence
techniques
thatprobe
the movementof,
forexample,
thephenyl rings
are of limited value intesting
the model. In fact the modelrequires
that thevarious
parts
of the molecule differ in their freedom of rotation. The molecules will often possess a third centraldipole
moment which is still assumed to berandomly
distributed. If this is not the case additionalphase
transitions arepredicted, leading
to otherphases
that are two-dimensional ferroelectrics within the smecticlayers.
Wulf
[6]
hasgiven
a model of theSc phase
in whichthe
repulsive,
orsteric,
forcesplay a
dominant role.The characteristic order is assumed to be
mainly
aresult of the effect of the molecular
shape
on thepacking problem
for theliquid.
In the case of theSc phase
the relevant factor is thezig-zag
grossshape
of the
molecules, thought
to be a result of end chains that aresymmetrically
attached to themolecules,
andare not collinear with the central
body
of the molecules(see Fig. 1 b).
The model calculation startsby writing
down an effective interaction between the molecules that
simulates,
at leastqualitatively,
the effect of themolecular
zig-zag shape.
This interaction is then used in a mean fieldcalculation, assuming
that theSA
orderis well established. Let u 1. u2 and
u3
be unit vectors in amolecule-fixed coordinate
system, u3 being along
thelong
molecular axis. Thezig-zag
interaction betweena
pair
of molecules 1 and 2 is taken to be[6]
The first term accounts for the fact that the molecules interfere less with one another if their
long
andshort axes
align together. Consequently,
the result-ing Sc phase
will be biaxial if the constituent moleculesare biaxial. In this model there is no
completely
freerotation of the molecules around their
U3
axis in theSc phase.
The second term inequation (6)
repre- sents the additionaltendency
of the molecules to tiltover with respect to the intermolecular vector r 12 ; r2 is the range of
A2(r12).
We mustrequire
that0
A2(r) AI(r)
in order to ensure that themolecules do not tilt when the
long
and short axes arenot yet
aligned.
We shall not discuss the details of the model. A second-orderphase
transitionSA Sc
ispredicted
with a tiltangle ql growing continuously
from zero at the
transition,
andalways remaining
smaller than
Tr/4 [6].
In order todistinguish
betweenMcMillan’s
dipole
model and Wulfs steric model it will be necessary toinvestigate
in detail thetype
of molecules thatgive
anSc phase.
Both models areincompatible
withcompletely
free rotation around thelong
molecular axis.Finally
Priest[7]
hasgiven
a model of theSA Sc phase
transition
assuming
that there is an effective molecular second-rank tensor which isresponsible
for theorientational
phenomena
in the smecticphase.
Denot-ing
an element of this tensorby
qij, the average of qijover molecules in the
vicinity
of apoint
r can beintroduced :
One can
expand
the orientational interaction energy between two molecules in a series bilinear in Q. Withappropriate
values for theexpansion
coefficients asecond-order
phase
transitionSA Sc
can be obtained.The tilt
angle
varies as(TeA - T)1/2
as in the othermodels,
while a smallbiaxiality
is induced[7]. Contrary
to the
previous models,
thebiaxiality
is due to thesymmetry
of theSc phase
rather than theSc
tiltbeing
the result of a
tendency
to form a biaxialphase.
Notethat in
equation (7) Qij
may be uniaxial even if qij is biaxial.However,
if qij is alsouniaxial,
free rotation around thelong
molecular axis is not forbidden in theSc phase.
Priest did notgive suggestions
for thespecific
tensor qij to be considered.However,
thispoint
wasrecently
taken upby
Cabib and Ben-guigui [8],
who treated the molecules asaxially
symme- tricobjects
in both theSA
and theSc phase,
andconsidered the interaction between the components of the
dipole
momentsparallel
to thelong
molecular axis.Hence their model is
complementary
to McMillan’sdipole
model. In factthey
suppose that each molecule1268
has two
opposite dipoles along
thelong
axis. TheSc phase
is induced because the molecules tend to slidealong
each other due to the electrostaticinteraction,
thus
increasing
the distance between the molecular centres.A well-known case of an
SA SC phase
transition is found interephthal-bis-butylaniline [16] (TBBA),
where the tilt
angle
indeed grows withdecreasing temperature
from zero atTCA,
aspredicted by
all themodels
given
above. There arehardly
any othercompounds
for which thispoint
has beeninvestigated.
In some other cases the
Sc phase
is observeddirectly
below a N
phase. Usually
alarge
tiltangle
is thenobserved
(say 450), independent
oftemperature [17].
Formally
aNSc phase
transition may be describedby combining
models for theNSA
and theSA Sc phase
transitions in a situation where
TAN TeA.
It is clearthat the tilt
angle
cannot then be zero at theNSc phase transition,
and may beapproximately independent
oftemperature
if the curveof 03C8
versusT/T CA
saturateswith
decreasing temperature.
The maximum value of the tiltangle
in Wulfs and in Priest’s model(450
and49.1 °, respectively)
is of theright
order ofmagnitude.
In McMillan’s model the maximum tilt
angle depends
on the details of the molecules.
3. Smectic
phases
ofalkyl-
andalkoxy-substituted
azobenzenes. - First we shall discuss the nature of the
mesophases
found in thecompounds
of the seriesThe transition temperatures were determined with a
Leitz
Orthoplan polarizing microscope equipped
witha Mettler FP52
heating
stage. Heats of transition wererecorded
by
means of differentialscanning calorimetry
with a Perkin-Elmer DSC IB. The results for series I and some of the
higher
members of series II aregiven
in tables I and II and
displayed
infigure
2. Thecompounds
of series I have also been discussed in reference[9],
but withoutexplicit
reference to thesmectic
phases.
The transition temperaturesgiven
TABLE I
Phase transitions
of
series I(K
standsfor crystalline ; monotropic
transitions areplaced
betweenparentheses)
e) Due to crystallization no quantitative measurement was
possible.
TABLE II
Phase transitions
of’series
IIe) Gabler, ref. [ 18], gives for this compound a monotropic SN transition at 97 °C. We could not reproduce this result, although the
N phase could be supercooled down to 94 °C. In some cases we observed a metastable cryitalline phase in the region 950-1000, which could
probably be mistaken for a smectic phase. This idea is in agreement with the fact that Gabler did not observe a smectic phase for n = 9.
FIG. 2. - Transition temperatures versus chain length for series I and II (for series II, n 6, from reference [18]).
FIG. 3. - Transition temperatures versus chain length for series III
(for n 5 from reference [24]).
here should be considered as more accurate. For the
higher
members of seriesI, SA phases
occur in additionto the N
phases.
This iseasily
established from thesimple
focal-conic orhomeotropic
textures and theoccurrence of one
sharp X-ray
reflection at smallBragg angle
in apowdered sample [19].
For n = 9and n = 10 an additional
SB phase
is found. The textures of thisphase
are either blurred focal-conic orhomeotropic,
the latteragain indicating uniaxiality.
In the
powder X-ray
diffractionpattern
twosharp
reflections are observed
(one
atsmall,
the other atlarge Bragg angle).
This classification of theSB phase
of(I, n
=9)
has been confirmed from itscomplete miscibility
with theknown SB phase
ofN-(p-n-pentyl- benzylidene) p’-n-hexylaniline [20].
For the
higher
members of seriesII, Sc phases
areobserved below the N
phases.
Under thepolarizing microscope
either broken focal-conic textures orschlieren textures are observed. The absence of inter- ference colours in the schlieren textures indicates a
relatively large
tiltangle directly
below theNSc
transi-tion. The classification of the
Sc phase
of(II, n
=10)
has been confirmed from its
complete miscibility
withthe known
Sc phase
ofp,p’-di-n-heptyloxyazoxy-
benzene
[21].
It is
interesting
to compare these results with those for thecorrespondingly
substitutedazoxybenzenes.
The
mesophases
of thep,p’-di-n-alkylazoxybenzenes
are described in reference
[22].
The smecticphases
ofthe
higher homologues
of this series are allSA (simple
focal-conic or
homeotropic
textures,complete
misci-bility
with theSA phase
of seriesI).
Themesophases
ofthe
p,p’-di-n-alkoxyazoxybenzenes
are described inreferences
[18]
and[23].
The smecticphases
of thehigher
members of this series are well known to be of theSc
type[21].
Hence we conclude thatreplacement
of the azo
linkage by
an azoxylinkage,
thus introduc-ing
a centraldipole
moment, does not have any influence on the type of smecticphases
that occur inthese systems.
TABLE III
Phase transitions
oj’series
III1270
Next we consider the
mesophases occurring
in theseries
Although
a strongdipole
is found in this seriesonly
atone
end,
the molecularshape
is stillapproximately symmetric.
The results for some of thehigher
membersof this series are
given
in table III andfigure
3. Forn = 8 and n =
9,
oncooling
from the Nphase,
an
SA phase
is firstobserved,
then anSc phase.
Theenthalpy
of theSA Sc
transition is very small. The tran- sition is best observed oncooling
ahomeotropic SA
texture. At the
SA Sc
transition a schlieren texture appears with interference coloursindicating
a tiltangle
that growscontinuously
from zero. This isconfirmed
by conoscopic
measurements where the maltese cross observed in ahomeotropic SA sample
moves off-centre when the
SA Sc
transition ispassed.
For n = 9 one observes on
cooling
a transition to athird smectic
phase
that was classified asS,.
In order to
investigate
whether theasymmetric shape
of the molecules affects certainmesophases,
wefinally
consider the seriesThe various
phases
of some of thehigher
members ofthe series are indicated in table IV and
figure
4. Theresults are very similar to those found for series III.
TABLE IV
Phase transitions
oj’series
IVFIG. 4. - Transition temperatures versus chain length for series IV
(for n 7 from reference [24]).
Note, however,
that for(IV, n
=8)
there is noSA phase;
theSc phase
goesdirectly
over into the Nphase.
For n = 9 an intermediateSA phase
appears.The
temperature
range in which theSA phase
isstable increases with
increasing
chainlength.
All themesophases
of series IV have textures similar to thoseof the
corresponding mesophases
of seriesIII,
withwhich
they
are alsocompletely
miscible. From theshift of the
conoscopic
cross observed inhomeotropic samples
the tiltangle
has been calculated for(IV, n
=11)
in thevicinity
ofTCA;
the results aregiven
infigure
5. The numericalaperture
of theconoscope was
only
0.33 as determinedby
the conden- ser,corresponding
to anangular
field of view of about 400 in air. The absolute value of the tiltangle depends
on the value of the maximum index of refrac-FIG. 5. - Tilt angle versus relative temperature in the Sc phase
of compound (IV, n = 11).
tion,
which was assumed to be 1.7. The variation oftilt with temperature around
T CA,
asgiven
infigure 5,
is very similar to that found in the well-known caseof TBBA.
In
general
the transitionSC SB
isonly
visible if theSc phase
is in a schlieren texture. Withdecreasing temperature
a new schlieren texture then appears at thetransition,
which isbrighter
and has fewersingularities.
In order tostudy
this third smecticphase
in more detail we made a mixture of 50 per cent(by weight)
of(III, n
=9)
and(IV, n
=11).
Thetransitions of this mixture are
approximately K30SB49Sc61SA73N80,
and theSB phase supercools easily
down to roomtemperature.
TheSB phase
inthis mixture also occurs as a blurred focal-conic texture that
gradually
tends tochange
into a mosaictexture. The
powder X-ray
diffractionpattern
containstwo
sharp
reflections without any additional structure.Hence we conclude that the classification of this
phase
as anSB phase
is correct. The occurrence of schlieren textures and the absence ofhomeotropic
textures indicates that this
SB phase
isprobably
biaxial.
4. Discussion. - We shall first discuss the results for series I and II. The
replacement
of aCH2
groupby
an oxygen atom has the effect of
introducing
adipole
moment of about 1.3
D,
at anangle
of about 720 withthe p,p‘
axis of theadjacent
aromaticring [25], giving
adipole component
of about 0.4 Dalong
thep,p’
axis.In the case of an
alkyl
group there is adipole
momentof 0.4 D
along
thisp,p’
axis. Hence thedipole
compo- nentsalong
thelong
molecular axis are very similar for thecompounds
of series I and II. AsSA phases
occurin one series and
Sc phases
in theother,
the model of Cabib andBenguigui
cannot beexpected
toapply
tothese systems. Furthermore the molecules of series I and II have a very similar molecular
shape.
An oxygen atom is somewhat smaller than aCH2
group[26],
which may make the molecules of series II about 0.5
A
shorter than the
corresponding
ones of series I.Moreover the
Car
CCangle
of 1080(tetrahedral value)
is
replaced by
aCar
OCangle
of 120° which may lead toa
slightly
morepronounced zig-zag shape
for series I.This difference between the series is reinforced
by
thefact that the
mesophases
of series I occur at lowertemperatures,
thusdecreasing
theflexibility
of the endchains in series I as
compared
with series II. Thisflexibility
can beexpected
to counteract thezig-zag
form. Hence if these differences are
important
atall,
it leads to a more
pronounced zig-zag
form for the molecules of series I than for series II. As theSA phases
occur in series I and the
Sc phases
in series II it isunlikely
that this difference is due to achange
in therepulsions
between thezig-zag shaped
molecules.On the other hand when
going
from series I to series IItwo outboard
dipole
moments are introduced. Hence the results are at leastqualitatively
consistent with McMillan’sdipole
model of theSc phase.
The factthat an additional central
dipole
moment has noinfluence on the
type
of smecticphases (substitution azo-azoxy) requires
that the central aromatic cores of the molecules still rotaterelatively freely
in these sys- tems. It isonly
for thedipoles
on the oxygen atoms that this rotation is not allowed. Thetendency
to forman
Sc phase
is strong for seriesII ;
there is noSA phase
intermediate between the N and the
Sc phase.
As soonas the
layered
structure is established thephase
takesthe form of an
Sc phase
with arelatively large
tiltangle.
An
interesting
test on thedipole
model of theSc phase
isprovided by
the results for seriesIII,
wherea weak
tendency
to form anSc phase
is found(SA phase
intermediate between N and
Sc phase,
tiltangle growing
withdecreasing temperature
from zero at theSA Se transition).
In this series astrong dipole
isavailable
only
at one side of themolecules,
while theshape
of the molecules of series I or II is retained.Assuming
that there is nopreference
for the asymme- tric molecules to be with thepolar
side up ordown,
McMillan’s model can still beapplied (see Fig. 6a).
However,
as the average distances between thedipoles
has been
increased, TcA
is reducedby
a factor 2J2.
This decrease of
TcA
is lesspronounced
if the induceddipole
moments due to the transversepolarizabilities
are taken into account.
If,
forsimplicity,
the transversepolarizability
of the molecule is assumed to berepresented by
twopoint polarizabilities
a atposi-
tions ±
d/2, equation (4)
must bereplaced by
where
n2
=n2/2. Using
Boo = 2.5(Ref. [27])
andn2 N 4 x
1014 (Ref. [5])
we findwhile a can be
expected
to be of the order of 1 x10- 23 cm3 [27].
Hence the effect of the inclusion of a is an increase ofTCA by
about 50%.
In the case of one end
dipole only,
we must alsoconsider the alternative situation of a
phase
that is aFIG. 6. - The two possibilities for dipole interaction in the case of
one outboard dipole moment only; in situation (b) the smectic
layers are two-dimensional ferroelectrics.
1272
two-dimensional ferroelectric within the smectic
layers (see Fig. 6b).
In the context of thepresent simple
models it is not useful to compare the relative
stability
of the
Sc phases depicted
infigure
6a andfigure 6b,
which in
general
willdepend
on the ratio between theasymmetric dipole potential
and thesymmetric
part of the total intermolecularpotential.
Wesuggest
thatfigure
6bprovides
apossible
model for theSF phase.
Like the
SF phase
the model has thephysical properties
of the
Sc phase.
In addition it will be ferroelectric oranti-ferroelectric, depending
on thesign
of the inter-planar
interaction. Thecompounds
studied here do not possess such an additionalphase.
These ideas would have to be tested oncompounds showing
anSc
andan
SF phase [10],
which areunfortunately
noteasily
available.
Finally
we come to the effect which thesymmetry
of theshape
of the molecules has on the formation of smecticphases.
Whencomparing
series IV with series III we first consider some isometriccompounds
that have the same number of
CH2
groups but adifferent
shape.
Compare
forexample :
We see that in
compounds
of the samelength
thetendency
to form a smecticphase
is greater in the case of a lesssymmetric shape.
This conclusion was also arrived atby
Malthete etal.,
who studied several isometric series in detail[28].
Anexplanation
for thiseffect has not yet been
given.
From tables III and IV wesee that there is no difference between the
type
of smec- ticphases
that occur in series III and IV. Inparticular
the
suggestion
thatSc phases
arepreferentially
foundin
symmetrically
substitutedcompounds [28, 29]
is notconfirmed, although
the results for series IV withincreasing n
indicate that if the deviation from symmetryincreases,
thetendency
to form a smecticphase
of some other type increases morestrongly
than the
tendency
to form anSc phase.
5. Conclusion. - We have shown that
alkyl and/or alkoxy
substituted azobenzenes mayshow,
besidesthe N
phase, SA
orSc phases
orboth, depending
on theend substituents. The
results,
summarized in tableV,
suggest that therepulsions
between thezig-zag shaped
molecules do not
play a
dominant role in the formation of theSc phase.
The results are at leastqualitatively
inagreement with McMillan’s
dipole
model of theSc phase, provided
theasymmetric
molecules of series III and IV have nopreference
forbeing
up or down. Otherwise the modelgives
a ferroelectric oranti-ferroelectric
phase
that couldpossibly
be iden-tified with the
SF phase.
TABLE V
Summary oj’ the
resultsAcknowledgments.
- The author wishes to thank Dr. J. Van der Veen formaking
thecompounds
ofseries I and II available to
him,
and Mr. J. Boven for thesynthesis
of thecompounds
of series III and IV.References [1] STEPHEN, M. J. and STRALEY, J. P., Rev. Mod. Phys. N 6 (1974)
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